fix Insert<T> when T is declared as IEnumerable<T>

[bdrupieski]

When inserting, when checking if the type implements IEnumerable<>, we weren't checking that the passed type is itself declared as IEnumerable<>. IEnumerable<> doesn't implement IEnumerable<>; it is IEnumerable<>.

This PR changes adds a check for if the type is itself IEnumerable<> in addition to if it implements IEnumerable<>.

This fixes #930.

[NickCraver]

Thanks for the fix!

[jons-aura]

Is there a release planned? I'm pretty sure this is what is breaking inserting lists of objects with Dapper.Contrib for me. I get

 {System.InvalidOperationException: An enumerable sequence of parameters (arrays, lists, etc) is not allowed in this context
   at Dapper.SqlMapper.GetCacheInfo(Identity identity, Object exampleParameters, Boolean addToCache) in C:\projects\dapper\Dapper\SqlMapper.cs:line 1686
   at Dapper.SqlMapper.QueryMultipleImpl(IDbConnection cnn, CommandDefinition& command) in C:\projects\dapper\Dapper\SqlMapper.cs:line 1012
   at Dapper.SqlMapper.QueryMultiple(IDbConnection cnn, String sql, Object param, IDbTransaction transaction, Nullable`1 commandTimeout, Nullable`1 commandType) in C:\projects\dapper\Dapper\SqlMapper.cs:line 998
   at SqlServerAdapter.Insert(IDbConnection connection, IDbTransaction transaction, Nullable`1 commandTimeout, String tableName, String columnList, String parameterList, IEnumerable`1 keyProperties, Object entityToInsert) in C:\projects\dapper\Dapper.Contrib\SqlMapperExtensions.cs:line 784
   at Dapper.Contrib.Extensions.SqlMapperExtensions.Insert[T](IDbConnection connection, T entityToInsert, IDbTransaction transaction, Nullable`1 commandTimeout) in C:\projects\dapper\Dapper.Contrib\SqlMapperExtensions.cs:line 355
   at MMA.Data.MMA.Dapper.MMAProvider.Create(IEnumerable`1 parts) in C:\_source\Client\application\MMA.Data\MMA.Dapper\MMAProvider.cs:line 69
   at MMA.Business.Imports.PartImport.ImportParts() in C:\_source\Client\application\MMA.Business\Imports\PartImport.cs:line 80}

[bdrupieski]

@NickCraver When this is included in a release I think we should also include #887 so that insert, update, and delete have consistent behavior.